Natural phosphate rock, particularly the mineral, apatite (calcium phosphate), is a primary commercial source of phosphorous. One of the most common methods of producing phosphoric acid from phosphate rock is the acid or wet process. The wet process comprises reacting refined phosphate rock with sulfuric acid to produce phosphoric acid and an impure calcium sulfate, known as phosphogypsum. Phosphogypsum has, until recently, been considered a waste product of the wet process, having no commercial value. Thus, great mounds of phosphogypsum have accumulated near and around phosphoric acid plants. These mounds of phosphogypsum pose an environmental problem due mainly to the acidification of rainwater and subsequent runoff of the soluble compounds from the phosphogypsum.
One commercially valuable process for the conversion of phosphogypsum into useful products is disclosed in U.S. Pat. No. 4,503,018 issued to Gardner et al. ("Gardner"), which is incorporated herein by reference in its entirety The Gardner process yields a sulfur-containing gas stream resulting from the thermal decomposition of the gypsum. More specifically, the Gardner process involves charging a pelletized mixture of carbonaceous material and gypsum to a travelling grate where the mixture is dried and sintered to produce a gaseous effluent containing sulfur dioxide and/or sulfur. After the pellets have undergone thermal decomposition, the lime residue may be sold or used in conventional applications.
U.S. Pat. No. 4,744,969, which is incorporated herein by reference in its entirety, describes another process for the conversion of phosphogypsum into useful products. This described process simultaneously converts high-sulfur, low-BTU coal and gypsum into sulfur-free and sulfur-rich gas streams along with a solid aggregate. The coal is gasified producing char and sulfur-containing gas from which sulfur is removed, leaving a substantially sulfur-free gas stream, and the sulfur is converted into solid sulfur compounds. These solid sulfur compounds along with the char and the gypsum are fed to a desulfurization reactor. The desulfurization reactor has predrying, drying, firing and cooling stages which convert the char and gypsum into the desired products.
Because of the rapid rate at which gypsum is produced in phosphoric acid plants, and the large stockpiles of this material which presently exist, there is a continuing need for more efficient gypsum-converting processes. If existing processing technology were employed for consuming all of the phosphogypsum produced, such would result in the production of excess sulfuric acid.